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Abstract:

A method in a mobile station including receiving an order from a serving
cell for system information acquisition of a neighbor cell, wherein the
order includes at least a physical cell identifier and a time limit for
acquisition of the system information of the neighbor cell, acquiring the
system information of the neighbor cell within the time limit for the
acquisition of the system information, and reporting at least a part of
the acquired system information to the serving cell.

Claims:

1. A method in a mobile station for acquiring neighbor cell system
information, the method comprising: determining, at the mobile station,
that a transmission of a packet on a wireless communication link between
the mobile station and a serving cell overlaps, at least in part, a time
window in which a neighbor cell performs a transmission of a system
information message the transmission of the packet occurring on at least
one of n occasions; receiving, at the mobile station, the system
information message from the neighbor cell provided that the transmission
of the packet does not occur on a last m of the n occasions, m is less
than n.

2. The method according to claim 1 further comprising: determining that
the transmission of the packet is one of the last m of the n occasions of
the packet transmission, and receiving the transmission of the packet on
the wireless communication link between the mobile station and the
serving cell.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a divisional of U.S. patent application
Ser. No. 12/792,194, filed Jun. 2, 2010, which claims benefit under 35
USC 119(e) to U.S. Provisional Application No. 61/219547 filed on Jun.
23, 2009, the disclosures of which are hereby incorporated by reference
herein.

FIELD OF DISCLOSURE

[0002] The present disclosure relates generally to wireless communications
and more specifically to acquisition of system information of neighboring
cells.

BACKGROUND

[0003] The reading of neighbor cell system information by a mobile station
in a wireless communication network is beneficial in many situations. One
such situation arises in 3rd Generation Partnership Project (3GPP)
Long Term Evolution (LTE) networks in the presence of Home eNBs (HeNBs),
which are also referred to as femto cells. An LTE HeNB is a low power eNB
that provides coverage in hotspots, e.g., in homes, etc., to a small
number of users. A physical cell identifier (PCID) uniquely identifies
each macro cell, since the network is planned and deployed such that
macro cells with the same PCID are separated by a very large distance to
facilitate re-use of PCIDs. In contrast, HeNBs are deployed by users in
an uncoordinated fashion with no PCID planning. As a result it is
possible to have two HeNBs under the coverage of a macro cell that use
the same PCID, which may result in PCID confusion.

[0004] For handover, the network needs to know precisely to which cell to
handover a mobile station. One approach to resolve PCID confusion is for
the mobile station to read the unique cell identifier (cell global ID or
CGI) or CSG ID in System Information block 1 of the target HeNB and to
report the CGI or the CSG ID. However, mobile stations in connected mode
in LTE Rel-8 are not required to acquire system information messages from
neighboring cells. Reading neighbor cell system information can cause
significant interruptions to an ongoing connection with a serving cell.
This interruption is particularly problematic when there is an ongoing
voice call. Thus it is desirable to reduce loss of data packets (e.g.,
voice frames) transmitted between a mobile station and a serving cell
when the mobile station attempts to acquire neighbor cell system
information.

[0005] The various aspects, features and advantages of the disclosure will
become more fully apparent to those having ordinary skill in the art upon
careful consideration of the following Detailed Description thereof with
the accompanying drawings described below. The drawings may have been
simplified for clarity and are not necessarily drawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1 illustrates a method for receiving system information
transmitted by a neighbor cell by shifting HARQ transmissions.

[0007] FIG. 2 illustrates a method for receiving system information
transmitted by a neighbor cell by using an additional HARQ process.

DETAILED DESCRIPTION

[0008] The various embodiments of the present disclosure minimize or at
least reduce the loss of packets when the mobile station tries to read
neighbor cell system information. In some embodiment, the HARQ behavior
on the communication link to the serving cell is adjusted for this
purpose. The disclosure relies on knowledge of the target cell's frame
timing to reduce the likelihood of missing HARQ transmissions to and from
the serving cell. The embodiments are described in terms of a 3GPP LTE
system. The system information messages of the neighbor cell that are
considered include the Master Information Block (MIB) and System
Information Block 1 (SIB1). However, it should be clear that the
teachings of the disclosure are equally applicable to other wireless
communication systems and other types of system information messages.

[0009] In some instances, the serving cell determines target cell
information, which is provided through the system information of the
target cell. For example, when a measurement report received from a
mobile station indicates that a target cell is a potential handover
candidate, the serving cell may determine that the target cell's cell
global identifier is needed

[0010] To obtain the target cell information, the serving cell sends a
system information acquisition order indicating or identifying the
physical cell identity (PCID) of the target cell for which additional
information is required. In some embodiments, this order includes a
system information acquisition time limit, wherein the mobile station is
expected to acquire the system information of the identified target cell
within the time limit.

[0011] In some embodiments, until the expiry of the system information
acquisition time limit, the serving cell uses a lower or reduced voice
codec rate for ongoing voice calls without lowering the resource
allocation. This reduces the size of the voice packets that are
transmitted and therefore minimizes the number of HARQ retransmissions
needed to successfully receive a voice packet. This in turn reduces the
number of lost voice packets due to the mobile station attempting to
receive system information messages from the target cell since the same
radio resources originally used for a larger packet is now used for a
smaller coded voice packet. For example, the serving network can use only
1/2 rate AMR voice during this period while the resource allocation
continues to be based on full rate AMR voice.

[0012] This technique can be applied to both uplink and downlink voice
packets, i.e., both the base station and the mobile station operate at a
reduced data rate. Furthermore, this technique can be applied to obtain
other system information as well. For example, such other system
information may include system information blocks 2-8 defined according
to the LTE Release 8 (Rel-8) specification in 3GPP TS 36.331.

[0013] Although the description above indicates that the serving cell
triggers the acquisition of the neighbor cell system information, it
should be clear that the mobile station can also autonomously trigger the
acquisition of the system information.

[0014] According to a second embodiment, it is assumed that the serving
cell knows the frame timing difference between its transmission and that
of the target cell. This information can be provided by the mobile
station in a measurement report, or the information may be provided by
the target cell by using signaling mechanisms available in the network.
Alternatively, the source cell and the target cell may be frame
synchronized implying a frame timing difference of zero between the
serving cell and the target cell. In LTE systems, since MIB transmissions
occur in sub-frame 0 and SIB1 transmissions occur in sub-frame 5, if the
serving cell knows the frame timing difference between itself and the
target cell, it can determine when the mobile station is likely to tune
to the target cell. Consequently, based on time overlap of HARQ
transmissions with transmissions of MIB or SIB1 in the target cell, the
serving cell can determine which HARQ transmissions could be missed by
the mobile station.

[0015] The sub-frames in the serving cell that overlap the transmissions
of MIB or SIB1 of the target cell are referred to as "unavailability
periods". According to this embodiment, the following technique is used
to minimize missed transmissions:

[0016] The serving eNB sends a system information acquisition order
indicating the PCID of the target cell. This order includes a system
information acquisition time limit. The mobile station is expected to
acquire the system information within this time limit.

[0017] Within the system information acquisition time limit a
"pre-unavailability time window" and a "post-unavailability time window"
are defined around each unavailability period. The pre-unavailability
time window consists of n1 sub-frames before the start of the
unavailability period and the post-unavailability time window consists of
n2 sub-frames after the end of the unavailability period.

[0018] The mobile station monitors for physical downlink control channel
(PDCCH) in the pre- and post-unavailability time windows.

[0019] If the timing of a planned HARQ transmission is such that it occurs
during the unavailability period, then it is instead transmitted during
the pre- or post-unavailability time windows. Note that this principle
can be applied to both downlink transmissions and uplink transmissions.
As an enhancement, instead of replacing the planned transmission, an
additional transmission can be performed in the pre or post
unavailability time windows.

[0020] HARQ Ack/Nack behavior: To allow for easier implementation, the
feedback to a transmission in the pre- or post-unavailability period
could be assumed to be a Nack. In other words, it is not necessary to
assign Ack/Nack resources for the shifted transmission. As an
alternative, the Ack/Nack resources corresponding to the originally
planned transmission could be used. As another alternative, new Ack/Nack
resources can be assigned for the shifted transmission.

[0021] The mobile station attempts to acquire the necessary system
information from the target cell during the unavailability periods. If
successfully acquired, the mobile station may report a part or all of the
required acquired system information to the serving cell. Alternatively,
the mobile station can simply use the acquired system information for
internal processing purposes. For example, upon acquiring the CSG
identifier in SIB1, the mobile station may determine that it is not
allowed to access the target cell, and therefore not report the
information to the serving cell.

[0022] According to a third embodiment, it is assumed that the serving
cell knows the frame timing difference between its transmission and that
of the target cell. As in the second embodiment, this information can be
provided by the mobile station in a measurement report, or it may be
provided by the target cell by using signaling mechanisms available in
the network. Alternatively, the source cell and the target cell may be
frame synchronized implying a frame timing difference of zero. The
following steps are performed:

[0023] The serving cell sends a system information acquisition order
indicating the PCID of the target cell. This order includes a system
information acquisition time limit. The mobile station is expected to
acquire the system information within this time limit.

[0024] The mobile station is assigned an additional HARQ process with
corresponding resources for transmission of packets and transmission of
Ack/Nacks. This assignment of the additional HARQ process could be
included in the system information acquisition order. Note additional
HARQ process could be assigned for uplink transmissions and/or downlink
transmissions.

[0025] The additional HARQ process is used only if a planned HARQ
transmission coincides with an unavailability period. Specifically, if a
planned HARQ transmission coincides with an unavailability period, the
next transmission opportunity on the additional HARQ process is used
instead. Alternatively, if a planned HARQ transmission coincides with an
unavailability period, then a transmission opportunity on the additional
HARQ process that occurs before the planned HARQ transmission can be
used.

[0026] The additional HARQ process is suspended at the end of the system
information acquisition time limit.

[0027] As in the second embodiment, the mobile station attempts to acquire
the necessary system information from the target cell by receiving the
system information messages when they are transmitted by the target cell.
If the system information is successfully acquired, the mobile station
may report a part or all of the acquired system information to the
serving cell. Alternatively, the mobile station can simply use the
acquired system information for internal processing purposes. For
example, upon acquiring the CSG identifier in SIB1, the mobile station
may determine that the mobile station is not allowed to access the target
cell, and therefore the mobile station may not report the information to
the serving cell.

[0028] In the second and third embodiments described above, it is assumed
that the serving cell knows the difference between its frame timing and
the frame timing of the target cell (mobile station reports this frame
timing difference).

[0029] In the description of embodiments above, the reading of the system
information is triggered by an order from the serving base station. It
should be noted that it is equally possible to have the mobile station
trigger the reading of the system information.

[0030] According to a fourth embodiment, since the mobile station is aware
of the MIB and SIB1 transmission occasions of the target cell, it chooses
its unavailability periods based on status information related to the
HARQ processes. For example, if a MIB or SIB1 transmission in the target
cell overlaps a planned HARQ transmission in the serving cell and the
HARQ transmission is one of the first n transmissions of a packet, then
the mobile station attempts to receive the MIB or SIB1 of the neighbor
cell. If the planned HARQ transmission is not one of the first n
transmissions of a packet, then the mobile station receives the HARQ
transmission. This allows the mobile station to receive system
information messages when they overlap earlier transmissions of a packet
while allowing the packet to be still received in the later HARQ
transmissions.

[0031] While the present disclosure and the best modes thereof have been
described in a manner establishing possession and enabling those of
ordinary skill to make and use the same, it will be understood and
appreciated that there are equivalents to the exemplary embodiments
disclosed herein and that modifications and variations may be made
thereto without departing from the scope and spirit of the inventions,
which are to be limited not by the exemplary embodiments but by the
appended claims.

Patent applications in class Having a plurality of contiguous regions served by respective fixed stations

Patent applications in all subclasses Having a plurality of contiguous regions served by respective fixed stations